Adding textures is easy, they just need to be in the MSH folder for your project and you need to create tga.option files like the assets that shipped with the game have. Then in texture edit mode you select a tile (on the panel on the right) and then type the texture and detail texture filenames into the input box on the right side of the screen. When you hit enter, the tile you selected will turn into your texture (low res texture) and you can start painting terrain with it. You can use one texture for both, but TEXTURE you see when you are far away and DETAIL you see when you are close to it.

Creating textures is an art in and of itself beyond the scope of the tools, but if you do a google search I'm sure you'll find plenty of instruction on how to create good textures.

That's a good question I don't really have an answer for. All you really have to do is copy one that exists and rename it, but which one to copy is the question. What we do is just use one from an object that already exists that most closely matches the new object we are creating.

They really don't change much when they are the same type, for example a unit's texture tga.option isn't going to change across units and neither will the msh.option file, but the unit is made up of multiple objects and textures and across objects they are different. SO what you would do in creating a unit is look at all of the files that make up a unit and copy the right option file for the right component.